Do You Know How to Spell?

My bedtime conversation with Finn:

Finn: Do you know how to spell Finn? F-I-N-N Finn.
Me : Good job spelling. Now go to sleep.
Finn: Do you know how to spell wall? M-O-O-O wall.
Me: Finn, please go to sleep.
Finn: Do you know how to spell rocking chair? M-O-O-O rocking chair.

He continued spelling "gates", "curtains", "that" and "covers" and laughed hysterically after spelling each one like M-O-O-O.